A term that denotes the rate of water vapour transmission through a fabric. This movement of water vapour is driven by the difference in concentration of water vapour (or humidity) between the surface of the skin and the air outside the garment, as well as the physical resistance of the fabric layers. Usually quoted as gms of water vapour per metre squared of fabric. There are numerous tests to achieve a gms/m2 figure. The simplest involves heating a container of water covered with the fabric being tested. Some are as complex as to pass air over the fabric to mimic outdoor conditions.